Abstract

Parker Solar Probe has been proven as instrumental for the observation of young solar wind. During the encounters E-07 and E-08, PSP recorded the core plus beaming protons crossing the heliospheric current Sheet region. When the observed PSP data is plotted in the temperature ratio versus parallel beta space, it is found that the occurrence distributions for both the core and the beam protons are limited/bounded within outer boundaries, superficially similar to those observed in the near-Earth environment. We develop a dynamical model based on the combined quasilinear instability and collisional relaxation theories to explain why the observed core-beam protons are confined within such boundaries. Specifically, it is shown that the boundaries in the high-beta regime are well explained by the threshold conditions of proton-cyclotron and firehose instabilities, while the low-beta boundaries are defined by the collisional relaxations between core and beam proton components.